Re: [Qemu-devel] [PATCH] target/mips: Disable R5900 support

[Philippe Mathieu-Daudé]

Hi Fredrik,

On Wed, Nov 14, 2018 at 4:31 PM Fredrik Noring <address@hidden> wrote:
> On Tue, Nov 13, 2018 at 11:51:54PM +0100, Philippe Mathieu-Daudé wrote:
[...]
> > At some point while reading your reviews, I understood the R5900
> > patches introduced incorrect behaviors for the non-R5900 cpus. In this
> > case this patch wouldn't suffice.
>
> No, that was never the case and I'm not aware of any such problems.
> However, there is a refactoring series, and we have observed preexisting
> bugs in the MIPS emulation, unrelated to the R5900. The opcode decoder
> could also be improved, such as in asserting reserved instructions in
> more cases where opcodes are invalid, etc.

Yes, we figure that out with Richard Henderson while playing with the
decodetree script.

> > Hoping I misinterpreted your reviews, then this patch is OK.
> > With one of the suggested comments:
> > Reviewed-by: Philippe Mathieu-Daudé <address@hidden>
> >
> > Then we will fix this for the 4.0 release.
>
> What exactly needs to be fixed regarding the psABI? The relevant opcodes
> would need to stay, and not be prohibited and removed as Aleksandar has
> suggested, since such opcode removal breaks the psABI requirements.
>
> Finally, as Maciej explained in some detail, the document that Aleksandar
> just recently requested is known to not exist, for any MIPS implementation,
> so we are not going to make any progress on that either.

I think this was just bad timing with the QEMU release cycles, which
stressed Aleksandar which is the only MIPS maintainer.

Since the R5900 User is a new feature, no downstream distributions
ship it, so it is safer for the community to disable/delay it,
this means we have now 3 months to improve this code, while being more relaxed.

The QEMU community is big, we have different cultures, live in many
timezones, speak different languages, and beside all we are humans
with our daily lives :)
I guess we all sometime stress out, that's why I think, even if
disabling this feature is technically not required (as you seem to
suggest), it is sane to disable it and get back on track together.

I believe Aleksandar is paid to maintain the MIPS TCG/KVM codebase,
and the Malta/Boston boards, all code used by his employer, which is a
huge quantity of C code.
Now the project is community based, and I understand this as it is
open as contributions like yours, or boards not backed by the MIPS
shareholders/company.
The TCG maintenance is shared with Richard, but I think the rest is
too much for a single maintainer.
Somehow the community is failing here, putting too much pressure on Aleksandar.

Let's see how we can improve that during the next merge window,
hopefully the R5900 will be fixed, and eventually his little sister
R3900 will be joining too!

Regards,

Phil.